Traction drive elevators in the industry have traditionally been pre-set to operate at a maximum design speed during operation without any variation. In traction drive elevators, a series of ropes connected to an elevator car extend over a drive sheave (and one or more secondary sheaves) to a counterweight. The ropes may be connected directly to the car and counterweight or to sheaves coupled thereto. Lifting force to the hoist ropes is transmitted by friction between the grooves of a drive sheave and the hoist ropes. The weight of the counterweight and the car cause the hoist ropes to seat properly in the grooves of the drive sheave.
Traction drive elevators are typically designed to operate at a certain maximum speed, for example 500 fpm, based on the maximum load capacity of the elevator. However, conventional traction drive elevators never exceed the maximum speed even if the load in the car is less than capacity. Drive motors for traction drive elevators are designed to provide the power needed to obtain maximum speed. For example, the following equation may be used to calculate design power of a drive motor in an elevator system:                     HP        =                                            (                              1                -                                  (                                      cw                    ÷                    100                                    )                                            )                        ×            CAPA            ×                          VEL              design                                            33            ,            000            ×                          (                              EFF                ÷                100                            )                                                          (        1        )            wherein,    HP is power (in horsepower),            cw is the counterweight (as a % of the maximum car capacity)        CAPA is the maximum car capacity (lbs.),        VELdesign is the pre-set design velocity of the elevator (fpm), and        EFF is the efficiency of the elevator (%), which for example is 50-85% in geared systems and 80-95% in gearless systems.        
Conventional practice for traction drive systems has been to utilize a counterweight whose weight equals the empty weight of the elevator car plus 50% of the car's capacity. As an example, for a 3,000 lb. capacity elevator with an empty car weight of 4,000 lbs., the counterweight would weigh 5,500 lbs. In this arrangement, the power required to displace the elevator is at a maximum when the elevator car is either empty or filled to capacity. When the elevator is filled to one-half of capacity (such as 1,500 lbs. in the example given above) the power required to displace the elevator is at a minimum because the forces in the ropes on each side of the drive sheave are equal.
Passenger elevators must be designed to carry freight and as well as people of varying weights. Passenger elevator capacity is always calculated conservatively. Elevators, when volumetrically filled with people, are rarely operating with full loads even during peak traffic periods. The weight of the people in a fully loaded passenger elevator rarely if ever equals 80% of the design capacity. In most cases, an elevator that is so crowded that it will not accept an additional passenger has a load that is approximately equal to 60% of full load capacity.
Modern traction drive elevator systems utilize variable speed drives (VSD). These drives are designed to deliver a specified amount of current to the motor. Since current is directly related to power, the size of these drives are usually rated by current, power, or both. In addition to system software that limits maximum velocity of the car, the VSD also limits maximum velocity.
Modern elevator systems also now use load-weighing devices that can precisely measure the load in the car. Various approaches to load measurement are used, including load cells, piezoelectric devices, and displacement monitors. All of these systems can consistently calculate the load in an elevator cabin to within 1% of its capacity. For example, in an elevator with a maximum capacity of 2,000 lbs., it is possible to measure the load in the cabin within 20 lbs.
In some instances, the prior art has used variable speed drives to control the motion of elevator cars in response to the load carried by the car. For example, U.S. Pat. No. 5,241,141, issued Aug. 31, 1993, to Cominelli, shows an elevator system including variable speed motor controlled in response to a selected motion profile to effect desired operation of the elevator car. Multiple elevator car motion profiles are stored in the memory of the controller. Depending upon whether or not an occupant is present in the elevator car, the controller selects either a comfortable high quality ride profile having an increased flight time and lower acceleration and jerk rates or a high performance profile having a decreased flight time and higher acceleration and jerk rates. If no passengers are detected in the elevator car by sensing the weight of the elevator car and its occupants, and by sensing the lack of car calls, then the elevator car is free to be dispatched to a floor having a hall call at a high performance rate to minimize the flight time to reach that floor.
U.S. Pat. No. 5,723,968, issued Mar. 3, 1998, to Sakurai, discloses variable speed elevator drive system for automatically discriminating between large and small loads, and for adjusting a maximum cage speed (maximum output frequency) in accordance with the load. The system comprises voltage and current detection circuits and a CPU which discriminates between large and small loads from a value obtained by averaging a detected current. The system automatically adjusts the maximum output frequency by determining whether the elevator is running in a regenerative state or a power state. According to the patent, by making variable the current detection range and period, and using a first order lag filter time constant in averaging the current, an optimal maximum output frequency corresponding to the load may be selected to improve the operating efficiency even when fluctuations in the load are large.
The prior art, however, has not recognized or suggested improving the performance of a traction drive elevator system by determining if the car is in a partially loaded state for a particular trip (i.e., a state where the load on the motor is less than maximum) and utilizing the excess power of the drive motor to alter the velocity profile of the car on the particular trip. The method and apparatus of the present invention achieve this objective and are able to alter the velocity profile by increasing the top speed of the car, or by accentuating the acceleration or jerk rates during a particular the trip ultimately to reduce the time of the trip.